In order to evaluate a change in chemical state of a polymer material containing at least one diene rubber due to deterioration, in general, methods such as an infrared spectroscopy (FT-IR), nuclear magnetic resonance analysis (NMR), and X-ray photoelectron spectroscopy (XPS) are used. FT-IR or NMR provides a detailed examination of the chemical state, but gives bulk information and therefore is difficult to use to examine in detail the chemical state after deterioration which proceeds from the sample surface.
Under such circumstances, there has been a demand to provide a method for examining the deterioration proceeding from the surface; for example, Patent Literature 1 proposes a method including irradiating a polymer material with high intensity X-rays, and measuring X-ray absorption while varying the energy of the X-rays, to analyze deterioration of the polymer, wherein the method includes a NEXAFS (Near Edge X-ray Absorption Fine Structure) method which measures an X-ray absorption spectrum near the absorption edge of a specific target element by using high intensity X-rays.
In the NEXAFS measurement, a method called the electron yield method which detects a current flowing when a sample is irradiated with X-rays is frequently used, which means that the sample generally needs to be an electrically conductive material. In general, polymer materials are insulating materials; however, when the sample is a tire rubber composition, particularly for sidewalls, for example, conductive carbon black is contained in a large amount, which ensures that even a relatively thick sample, such as having a thickness of about 1 mm, has conductivity and thus can be measured.
However, since tires have been required to have better fuel economy in recent years, there is a trend to decrease the amount of carbon black even in a sidewall rubber composition. Additionally, in the case of a tread rubber composition, there is a trend to add silica as a reinforcing agent to further decrease the amount of carbon black. In this way, recent fuel-efficient rubber formulations are less likely to ensure conductivity of the sample, and therefore such a sample having a thickness of about 1 mm is difficult to measure by NEXAFS.
The foregoing Patent Literature 1 also proposes processing (cutting) a polymer material with a microtome to be 100 μm or less, preferably 500 nm or less, in order to achieve a measurement with high S/B and S/N ratios. However, since the deterioration of a tire proceeds from the surface, it is necessary to cut the outermost surface and measure the outer surface side, but it is difficult to cut the outermost surface with a microtome. Even if the outermost surface can be cut, it is also difficult to determine which side of the prepared sample is the outer surface side.
As described above, on the basis of the conventional methods, it is difficult to examine in detail the deterioration proceeding from the sample surface by performing NEXAFS measurement of a sample poor in conductivity, such as particularly a tire rubber composition with a low carbon black content.
Meanwhile, as the method for examining a change in chemical state of a rubber material including a rubber component such as a diene rubber, and in particular a change in chemical state occurring starting from the surface of the rubber material, such as deterioration, for example, the following X-ray-based techniques are known: X-ray photoelectron spectroscopy (XPS method) and the method which measures an X-ray absorption spectrum near the absorption edge of a specific target element by using high intensity X-rays (NEXAFS method), as described above.
The XPS method and the NEXAFS method are each a surface-sensitive measurement technique with a detection depth ranging from the surface to a few tens of nanometers. In these methods, to examine the chemical state of polymers, for example, measurements targeting carbon are in general performed (e.g., a spectrum near the 1s orbital of carbon in the XPS method, and a spectrum near the carbon K-shell absorption edge in the NEXAFS method).
However, it is difficult for the conventional evaluation methods to accurately measure a change in chemical state on the surface of a sample such as a deteriorated rubber material. Therefore, there is a demand to provide an evaluation method capable of accurately measuring a change of the surface condition.